As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which exhibit high energy density and operating potential, have long cycle lifespan, and have a low self-discharge rate, are commercially available and widely used.
In addition, as recent interest in environmental problems is increasing, research into electric vehicles (EVs), hybrid electric vehicles (HEVs), and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes behind air pollution, is underway. As a power source of EVs, HEVs, and the like, a nickel metal-hydride (Ni-MH) secondary battery is mainly used. However, research into lithium secondary batteries having high energy density, high discharge voltage, and high output stability is actively carried out and some of the lithium secondary batteries are commercially available.
A lithium secondary battery has a structure in which an electrode assembly, which includes: a cathode prepared by coating a cathode active material on a cathode current collector; an anode prepared by coating an anode active material on an anode current collector; and a porous separator disposed between the cathode and the anode, is impregnated with a lithium salt-containing non-aqueous electrolyte.
These lithium secondary batteries generally use metal oxides as a cathode active material, such as lithium cobalt-based oxides, lithium manganese-based oxides, lithium nickel-based oxides, and the like and carbonaceous materials as an anode active material, and such lithium secondary battery is manufactured by disposing a polyolefin-based porous separator between an anode and a cathode and impregnating the resultant structure with a non-aqueous electrolyte containing a lithium salt such as LiPF6 or the like. When the lithium secondary battery is charged, lithium ions of the cathode active material are deintercalated and then are intercalated into a carbon layer of the anode. When the lithium secondary battery is discharged, the lithium ions of the carbon layer are deintercalated and then are intercalated into the cathode active material. In this regard, the non-aqueous electrolyte acts as a medium through which lithium ions migrate between the anode and the cathode.
Such electrolyte basically requires stability within an operating voltage range of a battery, i.e., 0 to 4.2 V, and the capability to transfer ions at a sufficiently high rate. To satisfy the requirements, a mixed solvent prepared by appropriately mixing a cyclic carbonate compound such as ethylene carbonate, propylene carbonate, or the like, with a linear carbonate compound such as dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, or the like is used as an electrolyte solvent.
However, lithium secondary batteries have high operating potentials and thus high energy may instantaneously flow therein. Accordingly, when a lithium secondary battery is overcharged to 4.2 V or higher, the electrolyte starts to decompose, and when the temperature of the electrolyte increases, the electrolyte may readily reach an ignition point, which results in high possibility of combustion.
In addition, recently, instead of using conventional materials as electrode active materials, research into use of spinel-structure lithium manganese composite oxides as cathode active materials or use of lithium metal oxides, e.g., lithium titanium oxides as anode active materials is underway.
In particular, among lithium manganese composite oxides, spinel-structure lithium nickel manganese composite oxides having formula LixNiyMn2-yO4, where y=0.01 to 0.6, which are used as active materials for high-voltage applications since they have an average voltage of 4.7 V, reach oxidation potential of an electrolyte and thus the electrolyte is oxidized, resulting in generation of by-products such as gas and the like, which deteriorates secondary battery safety.
Therefore, there is a need to develop an electrolyte that does not cause these problems, is stable at high voltages, and improves rate characteristics.